A Study of the Effect of Light on the Structural and Electronic Properties of Propylene Glycol

Abstract

In this study, the structural, electronic, optical, and spectroscopic properties of propylene glycol in its liquid state were investigated by using ethanol as a solvent. The analysis was conducted through Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT), employing the B3LYP functional in combination with the 6-311+G­(d,p) basis set. Calculations were performed using Gaussian 09 and GaussView 6.0 software, chosen for their high reliability and accuracy in molecular modeling. Optimized geometries of propylene glycol in both the ground state (prior to light exposure) and the excited state (following light exposure) were obtained to identify structural changes induced by photoexcitation, including variations in bond lengths and bond angles. The electronic properties were subsequently calculated to assess the chemical reactivity of the molecule. Furthermore, infrared (IR) and ultraviolet–visible (UV–Vis) spectra were simulated and compared with the experimental data. The strong agreement between theoretical and experimental results confirms the validity and reliability of the computational approach employed.

Introduction

Photochemistry is a branch of chemistry that explores the chemical effects of light on matter. It primarily refers to reactions initiated by the absorption of electromagnetic radiation, particularly ultraviolet light (100–400 nm), which is subdivided into three regions: UVC (100–280 nm), UVB (280–315 nm), and UVA (315–400 nm). In addition to ultraviolet light, visible light (400–750 nm) and infrared radiation (750–2500 nm) also contribute to photochemical processes.

These light-induced reactions play a vital role in both natural and industrial contexts, influencing phenomena such as photosynthesis, atmospheric transformations, and the breakdown of environmental pollutants. A deep understanding of how light interacts with molecules at the electronic level is essential for advancing technologies in materials science, photodynamic therapy, and solar energy conversion.

Harnessing light to catalyze chemical reactions is regarded as one of the most efficient and selective strategies in contemporary chemistry. Upon absorption of a photon, a molecule undergoes changes in its electronic configuration, which alters its reactivity and interactions with other species. This principle underpins numerous applications across various sectors, notably in the pharmaceutical and cosmetic industries.

Cosmetic products, in particular, exemplify the practical integration of chemistry into daily life. They typically consist of complex formulations containing 10 to 15 distinct chemical compounds. Among these, propylene glycol emerges as a key ingredient due to its versatility and widespread use in both cosmetic and pharmaceutical preparations.

Propylene glycol exhibits a range of physicochemical properties that underscore its significance in both pharmaceutical and cosmetic formulations. It functions as a humectant, effectively retaining moisture, enhancing formulation texture, and facilitating the penetration of active ingredients into deeper layers of the skin. Additionally, its antimicrobial activity contributes to the preservation of products by preventing contamination and spoilage. ,

Beyond topical applications, propylene glycol serves as a solvent in oral and injectable medications, promoting the absorption of active compounds on the skin. It also plays a stabilizing role, helping to prevent phase separation and degradation of formulation components. ,

In 2013, absorption bands corresponding to the vibrational modes of specific functional groups in propylene glycol were identified, providing insight into its molecular behavior. However, critical questions remain regarding its photostability: What impact does light exposure have on this essential compound? Could it undergo degradation or structural alterations under illumination? Might such changes affect its electronic or physicochemical properties?

Given that the degradation of any compound within cosmetic or pharmaceutical products may pose risks to skin or hair health, investigating the photochemical behavior of propylene glycol is necessary and timely.

Despite its widespread use in pharmaceutical and cosmetic formulations, the photophysical properties of propylene glycol remain largely unexplored. It is often assumed that this compound is stable under light exposure; however, the validity of this assumption has not been rigorously tested. This gap in knowledge underscores the need for a comprehensive quantum chemical investigation employing advanced theoretical and computational techniques to evaluate the compound’s behavior upon illumination.

Accordingly, the present study aims to examine the influence of light on the structural and electronic properties of propylene glycol, elucidate its optical characteristics, and determine its infrared and ultraviolet–visible spectra. By doing so, the research seeks to provide deeper insight into the compound’s photostability and potential transformations, which may have implications for its safe and effective use in light-sensitive applications.

Subjects and Methods

Quantum-Chemical Method

Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations were performed using the B3LYP , functional, which is widely recognized for its reliability in describing long-range interactions and noncovalent effects. The extended valence basis set 6-311+G­(d,p) was employed, where (d,p) denotes polarization functions and (+) indicates diffuse functions. Accordingly, the computational level of theory is represented as DFT/B3LYP/6-311+G­(d,p). All calculations were carried out with the Gaussian 09 , package in combination with Gauss View 6.0.

To realistically simulate the liquid phase environment, ethanol was included as the solvent using the Solvation Model based on Density (SMD). This implicit solvation model incorporates the dielectric constant of ethanol and provides a realistic description of bulk solvation effects by accounting for both electrostatic and nonelectrostatic contributions. Although explicit hydrogen-bonding interactions with individual solvent molecules were not directly modeled, their influence is partially captured within the SMD framework. As a result, the computed excitation energies and spectral profiles adequately reflect the essential role of solvent polarity while acknowledging that specific hydrogen-bonding interactions may introduce additional fine shifts in experimental spectra.

Calculations

Using the Gaussian 09 program, calculations were carried out on the atoms (hydrogen, carbon, oxygen) and on the molecule propylene glycol consisting of the previous atoms; for this, it was necessary to input the coefficients for the atoms or the studied compound through graphical interfaces, which are independent programs, and here, the graphical interface used was Gauss View 6.0.

The dissociation energy (D _e) of the molecule (M) has been calculated as follows:

$${\mathit{D}}_{\mathrm{e}}(\mathrm{M})=\sum _{\mathit{A}=1}^{\mathit{N}}{\mathit{E}}_{\mathit{i}}(\mathrm{A})-{\mathit{E}}_{\mathit{i}}(\mathrm{M})$$ 1

where E _i (A) is the total electronic energy of an atom A and E _i (M) is the total electronic energy of molecule M.

The energy gap (E _gap) is calculated as follows:

$${\mathit{E}}_{\mathrm{g}\mathrm{a}\mathrm{p}}=|{\epsilon }_{\mathrm{H}\mathrm{O}\mathrm{M}\mathrm{O}}-{\epsilon }_{\mathrm{L}\mathrm{U}\mathrm{M}\mathrm{O}}|$$ 2

where ε_HOMO is the energy of the highest occupied molecular orbital and ε_LUMO is the energy of the lowest unoccupied molecular orbital.

The absorption energy was determined by calculating the difference between the total electronic energy of the molecule in the ground state (before exposure to light) and that in the excited state (after exposure to light) according to the following equation:

$${\epsilon }_{\mathrm{Absorption}}={\mathit{E}}_{\mathit{i}}(\mathrm{o}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d}\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{e})-{\mathit{E}}_{\mathit{i}}(\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{s})$$ 3

The wavelength of the absorbed light (to excite the molecule) is calculated as follows:

$${\epsilon }_{\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}=\mathit{h}{\nu }_{\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}=\mathit{h}\frac{\mathit{c}}{{\lambda }_{\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}}\Rightarrow {\lambda }_{\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}=\frac{\mathit{h}\mathit{c}}{{\epsilon }_{\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}}$$ 4

h is Planck’s constant (h = 6.626 × 10^–27 erg·s = 4.1346 × 10^–15 eV·s) and c is the speed of light (c = 3 × 10¹⁰ cm·s^–1 = 3 × 10¹⁷ nm·s^–1).

The fluorescence emission energy is calculated by determining the energy of the first singlet excited state:

$${\epsilon }_{\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}^{\mathrm{F}}=E_{\mathit{i}}(\mathrm{optimized}\mathrm{ground}\mathrm{state})-E_{\mathit{i}}(\mathrm{optimized}\mathrm{first}\mathrm{excited}\mathrm{singlet}\mathrm{state})$$ 5

The wavelength of fluorescence light emission is calculated as follows:

$${\lambda }_{\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}^{\mathrm{F}}=\frac{\mathit{h}\mathit{c}}{{\epsilon }_{\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}^{\mathrm{F}}}$$ 6

The phosphorescence emission energy is calculated by determining the energy of the first triplet excited state:

$${\epsilon }_{\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}^{\mathrm{P}\mathrm{h}}=E_{\mathit{i}}(\mathrm{optimized}\mathrm{ground}\mathrm{state})-E_{\mathit{i}}(\mathrm{optimized}\mathrm{first}\mathrm{excited}\mathrm{triplet}\mathrm{state})$$ 7

The wavelength of light phosphorescence emission is derived from the following equation:

$${\lambda }_{\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}^{\mathrm{P}\mathrm{h}}=\frac{\mathit{h}\mathit{c}}{{\epsilon }_{\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}^{\mathrm{P}\mathrm{h}}}$$ 8

The energy absorbed by the molecule and used for activation is calculated from the following equation:

$${\epsilon }_{\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}={\epsilon }_{\mathrm{a}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}-{\epsilon }_{\mathrm{emission}}^{\mathrm{F}}$$ 9

The quantum yield is calculated from the following equation:

$$\varphi =\frac{{\epsilon }_{\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}}{{\epsilon }_{\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}}$$ 10

The yields of fluorescence and phosphorescence are derived from the following two equations:

$${\varphi }^{\mathrm{F}}=\frac{{\epsilon }_{\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}^{\mathrm{F}}}{{\epsilon }_{\mathrm{Absorption}}}$$ 11

$${\varphi }^{\mathrm{P}\mathrm{h}}=\frac{{\epsilon }_{\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}^{\mathrm{P}\mathrm{h}}}{{\epsilon }_{\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}}$$ 12

Results and Discussion

The effect of light on the structural, electronic, spectral, and optical properties of propylene glycol was studied in the liquid state using ethanol as the solvent, and ethanol was chosen as the solvent because it has been shown to exert the least influence on the structural and electronic properties of compounds − based on the Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT).

Structural Properties

All geometry optimizations were performed using the default convergence thresholds in Gaussian 09. According to literature benchmarks, applying stricter criteria (e.g., energy change <10^–6 a.u., gradient norm <10^–5 a.u.) typically leads to negligible structural changes (<0.01 Å in bond lengths, < 0.2° in bond angles), values well within the intrinsic accuracy of DFT methods. , Similarly, the robustness of the SMD solvation model has been demonstrated in previous studies, where modest variations in solvent parameters (e.g., dielectric constant ± 5%) were shown to produce only minor effects on excitation energies (<0.05 eV) and dipole moments (<0.2 D), significantly smaller than the typical uncertainty of TD-DFT methods. These literature benchmarks support the robustness of the present results with respect to both the optimization criteria and solvent model parameters.

As a first step, the effect of light on the structural properties of propylene glycol was studied in the liquid state using an ethanol solvent. Figure shows the optimized 3D geometric structure of propylene glycol with atom labels and numbering.

1.

Optimized 3D geometric structure of propylene glycol.

The optimized 3D geometric structures of the propylene glycol molecule were obtained for the ground state as well as for the singlet and triplet excited states. Figure illustrates these steric structures along with Mulliken partial atomic charges Q and the bond length and bond angle values, as presented in Tables and .

2.

3D geometric structures of propylene glycol.

1. Bond Length Values in the Three States: Ground, Singlet Excited, and Triplet Excited .

B3LYP/6-311+G(d,p)
Bond lengths (Å)	The ground state	The singlet excited state	The triplet excited state
O4–H12	0.965	1.015	1.041
O4–C3	1.438	1.333	1.410
C3–H11	1.096	1.087	1.090
C3–H10	1.096	1.087	1.088
C3–C1	1.520	1.898	1.545
C1–C2	1.518	1.490	1.519
C2–H9	1.094	1.091	1.090
C2–H8	1.093	1.093	1.093
C2–H7	1.093	1.096	1.092
C1–H6	1.980	1.090	1.125
C1–O5	1.441	1.336	1.448
O5–H13	0.967	1.011	0.981

^aBond lengths in Å and bond angles in degrees. Note: Bold values in the third and fourth columns indicate bond dissociation.

2. Bond Angle Values in the Three States: Ground, Singlet Excited, and Triplet Excited.

B3LYP/6-311+G(d,p)
Bond angles (deg)	The ground state	The singlet excited state	The triplet excited state
H12–O4–C3	108.68	110.37	108.05
H11–C3–H10	108.92	118.19	110.19
O4–C3–C1	107.87	109.54	109.92
C3–C1–C2	112.14	104.84	113.10
H9–C2–H8	108.51	109.86	108.97
C1–C2–H7	110.30	107.37	110.84
O5–C1–H6	108.60	114.65	105.52
C1–O5–H13	107.51	110.39	109.92

As reflected in the variations of bond lengths and bond angles shown in Tables and , the hydroxyl groups (−OH) and adjacent C–H bonds are expected to be the most affected upon photoexcitation owing to the higher electronegativity of oxygen atoms and their role in electron density redistribution, which makes these moieties potential sites for photochemical interactions or hydrogen-bonding rearrangements. This interpretation is consistent with previous studies on small polyols. ,

Electronic Properties

After the optimized geometric structures were obtained and the bond lengths and angles were determined, the electronic propertiesincluding dissociation energy, dipole moment, HOMO and LUMO energies, and the energy gapwere calculated in the liquid state using ethanol as a solvent, both before and after exposure to light for propylene glycol, as shown in Table . In addition, the Mulliken atomic charges of propylene glycol in the liquid state (ethanol solvent) were evaluated before and after light exposure, and the results are presented in Table . This table reports the atomic charges for the ground, singlet, and triplet excited states, together with the corresponding percentage changes (E _rel %), thereby providing further insight into the redistribution of electronic density upon photoexcitation.

3. Electronic Properties of Propylene Glycol in the Liquid State Using Ethanol as a Solvent Before and After Exposure to Light.

B3LYP/6-311+G(d,p)
	Before exposure to light	After exposure to light		Percentage change (E rel%)
Electronic property	Ground state	Singlet excited state	Triplet excited state	Change between the ground and singlet excited states	Change between the ground and triplet excited states
D e(M) (eV)	52.0415	46.2374	45.3753	–11.15	–12.80
μ p (debye)	3.4438	1.2963	7.2641	–62.36	+110.93
E LUMO (eV)	0.2067	0.0408	0.2122	–80.26	–2.63
E HOMO (eV)	–7.6704	–5.3394	–1.4198	–30.39	–81.48
E gap (eV)	7.8776	5.3820	1.6339	–31.68	–79.25

4. Mulliken Atomic Charges of Propylene Glycol in the Liquid State (Ethanol Solvent) Before and After Light Exposure.

	Before exposure to light	After exposure to light		Percentage change (E rel%)
Atom	Ground state	Singlet excited state	Triplet excited state	Change between the ground and singlet excited states	Change between the ground and triplet excited states
C1	–0.009	–0.105	–0.890	–1066.67	–9788.89
C2	–0.454	–0.463	–1.034	–1.98	–127.75
C3	–0.253	–0.264	–0.277	–4.35	–9.49
O4	–0.453	–0.374	0.085	+17.46	+118.76
O5	–0.412	–0.353	0.654	+14.32	+258.74
H6	0.161	0.150	0.299	–6.83	+85.71
H7	0.159	0.145	0.188	–8.81	+18.24
H8	0.159	0.157	0.180	–1.26	+13.21
H9	0.158	0.160	0.191	+1.27	+20.89
H10	0.165	0.149	0.231	–9.70	+40.00
H11	0.167	0.150	0.230	–10.18	+37.72
H12	0.312	0.320	0.250	+2.56	–19.87
H13	0.299	0.327	–0.107	+9.36	–135.79

Figure shows the energy gap of propylene glycol, the energies of the HOMO and LUMO orbitals, and the change resulting from exposure to light.

3.

Energy gap of propylene glycol. (A) Ground state, (B) singlet excited state, and (C) triplet excited state.

Analysis of Figures and , together with extrapolated values from Tables –, reveals that exposure of propylene glycol to light leads to

• 1.Propylene glycol undergoes degradation, and some bonds are dissociated in the singlet and triplet excited states. In the singlet excited state, the C1–C3 bond is dissociated, with the distance between the two atoms increasing to 1.898 Å, and in the triplet excited state, the O4–H12 bond is dissociated so that the distance between the two atoms becomes 1.041 Å.
• 2.The bond dissociation energy decreases after light exposure, indicating that the compound dissociates more rapidly.
• 3.The computed ground-state dipole moment of propylene glycol (3.44 D) is slightly higher than the reported experimental gas-phase values (2.3–2.5 D), a difference that can be attributed to solvent polarization effects and the distinction between gas-phase and solution environments. Upon excitation, the dipole moment decreases to 1.30 D in the singlet excited state, indicating reduced polarizability, whereas it increases markedly to 7.26 D in the triplet excited state, reflecting significant charge redistribution.
• 4.The low energy of the LUMO orbital in the singlet excited state indicates the effectiveness of the compound and its tendency to gain electrons (oxidation property). However, an increase in LUMO energy is noted in the triplet excited state.
• 5.The energy of the HOMO orbital increases as a result of exposing the compound to light, meaning that propylene glycol becomes less stable and more chemically reactive.
• 6.A significant change in the energy gap is observed: light exposure causes a decrease in the energy gap by 31.68% in the singlet excited state and 79.25% in the triplet excited state, meaning that the light led to an increase in the reactivity of propylene glycol. This demonstrates that light strongly affects the properties of propylene glycol.
• 7.The calculated HOMO–LUMO gaps and dipole moments show significant differences between the ground and excited states (7.88 eV → 5.38 eV → 1.63 eV; 3.44 D → 1.30 D → 7.26 D for ground, singlet, and triplet states, respectively). Mulliken charge analysis quantitatively supports these trends, highlighting redistribution of electron density upon excitation. Together, these results confirm substantial reorganization of the electronic structure, consistent with the known photophysical behavior of small polyols.

Taken together, these results demonstrate that light irradiation induces profound structural and electronic changes in propylene glycol, significantly enhancing its reactivity and altering its photophysical behavior.

The total electronic energies of propylene glycol (E _i (M)), its cation (E _i (M)⁺), and its anion (E _i (M)⁻) in the ground state were calculated in the presence of ethanol as a solvent. Furthermore, key electronic descriptors were determined, including the ionization energy (VIP), electron affinity (E _A), electronic chemical potential (μ), electronic chemical hardness (η), and the nonlocal electronegativity index (biological activity coefficient) (ω). These parameters provide valuable insights into the molecule’s reactivity, stability, and potential biological interactions. All computational results are systematically summarized in Table .

5. Total Electronic Energies of Propylene Glycol and Its Ions, along with Ionization Energy, Electron Affinity, Chemical Potential, Chemical Hardness, and the Biological Activity Coefficient.

Electronic property	Value, eV	Electronic property	Value, eV
E i (M)	–7335.3528	η	3.0340
E i (M) +	–7328.7589	μ	–3.5598
E i (M) –	–7335.8787	ω	2.0883
VIP	6.5939	–	–
E A	0.5258	–	–

Spectral Properties

IR Spectra of Propylene Glycol

As a second step, the absorption spectra of propylene glycol in infrared (IR), visible and ultraviolet (UV-Vis) regions were studied. Figure shows the computed quantum IR spectrum of propylene glycol (dissolved in ethanol). The absorption bands were then compared with the experimental spectrum shown in Figure .

4.

IR spectrum of propylene glycol dissolved in ethanol quantum chemically.

5.

IR spectrum of propylene glycol in the solid state experimentally.

Table presents the vibrational frequencies of certain functional groups in propylene glycol based on quantum-chemical, experimental, and reference IR spectra.

6. Absorption Bands for the IR Spectrum of Propylene Glycol with Assigned Vibration Types.

	Propylene glycol
	Wavenumber (cm–1)
Functional Group	Com.	Exp.	ref.
O–H stretch	3816 and 3781	3360 free	3300 free
C–H stretch	3096–2998	2995–2890	2980–2890
C–OH stretch	1021	1045	1040

^aComputed quantum-chemical DFT/B3LYP/6-311+G­(d, p) (this work).

^bExperimental (this work).

^cTaken from ref .

The results presented in Table indicate that the vibrational frequencies obtained from quantum chemical calculations are in close agreement with both experimental measurements and reference data. In particular, the O–H stretching vibration is observed in the experimental spectrum as a single broad band, which can be attributed to the formation of intermolecular hydrogen bonds. The pronounced absorption intensity of this band further reflects the strength of these hydrogen-bonding interactions, thereby reinforcing the reliability of the simulated spectra. It should also be noted that the computed spectra were generated within the harmonic approximation without explicit consideration of temperature or vibrational averaging effects. Such effectsespecially anharmonicity and hydrogen-bonding dynamicsare known to cause slight red shifts and band broadening in experimental spectra, most prominently in the O–H stretching region. Despite these limitations, the overall agreement between the simulated and experimental spectra provides strong confirmation of the validity of the computational assignments.

UV–Visible Spectra of Propylene Glycol

Electronic transitions have been studied based on the Time-Dependent Density Functional Theory (TD-DFT) using the B3LYP method and the extended basal group (6-311+G­(d,p)). When a molecule absorbs energy from ultraviolet or visible light, an electron transitions from a low-energy molecular orbital to a higher-energy orbital. The type of electronic transition was determined based on the molecular orbital diagram of the propylene glycol molecule, where the transition from the nonbonding orbital (n) to the antibonding σ orbital (σ*) is illustrated, in addition to the transition from the σ orbital (σ) to the antibonding σ orbital (σ*), as in Figure .

6.

Molecular orbital diagram of propylene glycol.

The UV–Vis absorbance of propylene glycol was investigated both quantum-chemically and experimentally (Figure ).

7.

UV–Vis spectrum of propylene glycol dissolved in ethanol quantum-chemically (a) and experimentally (b).

The quantum UV–Vis spectrum can be compared with the experimental, and we note a peak λ₁. The absorbed wavelengths corresponding to the electronic transitions are summarized in Table .

7. Absorbed Wavelengths of Propylene Glycol in the UV–Vis Spectra with Type of Electronic Transition.

	Propylene glycol
		Wavelength (nm)
No.	Electronic transition	Com.	Exp.
λ 1	n → σ*	189.9	190

^aComputed quantum-chemical DFT/B3LYP/6-311+G­(d,p) (this work).

^bExperimental (this work).

Optical Properties and Jablonski Diagram

In the third step, the optical properties of propylene glycol in ethanol were studied, such as absorption energies (ε_Absorption), fluorescence emission (ε_Emission ), phosphorescence emission (ε_Emission ), quantum yield (ϕ), and intersystem crossing. The absorption and emission wavelengths specifying the ranges of the spectra are listed in Table .

8. Optical Properties of Propylene Glycol.

B3LYP/6-311+G(d,p)
Optical property	Value	Optical property	Value	Area of spectrum
εAbsorption (eV)	6.5460	λAbsorption (nm)	189.53	UVC
εEmission (eV)	2.8367	λEmission (nm)	437.37	Visible (Blue)
εEmission (eV)	0.9333	λEmission (nm)	1329.35	IR
εActivation (eV)	3.2407	–	–	–
εIntersystem crossing (eV)	2.5070	–	–	–
ϕF	0.50	–	–	–
ϕPh	0.14	–	–	–
ϕ	0.49	–	–	–

Finally, the Jablonski diagram of propylene glycol is described in Figure where excitation takes place as a result of electronic transitions from the ground state to the excited states. The diagram also shows the processes of fluorescence, intersystem crossing, and phosphorescence within the molecule.

8.

Jablonski diagram of propylene glycol.

To excite the studied compound dissolved in ethanol, the sample should be irradiated with light of energy (ε_Absorption), wavelength (λ_Absorption), and frequency (ν _Absorption), as shown in Table .

9. Maximum Wavelength and Minimum Radiation Frequency for Propylene Glycol.

Optical property	εAbsorption (eV)	λAbsorption (nm)	ν Absorption (s–1)
Propylene glycol	≥6.5460	≤189.53	≥1.5828 × 1015

Summary and Conclusions

The structural, electronic, spectral, and optical properties of propylene glycol were studied through the following steps:

• 1.The optimized structures of propylene glycol were determined before and after exposure to light (singlet excited state and triplet excited state). Light exposure led to partial molecular degradation, with some bonds observed to dissociate.
• 2.We calculated and compared several electronic parameterssuch as bond dissociation energy, energy gap, and dipole momentbetween the ground and excited states. A decrease in these values was observed, indicating a significant effect of light on the molecule.
• 3.We calculated the quantum yield, fluorescence and phosphorescence yields, absorption energy, and activation energy of propylene glycol to show the extent to which this compound is affected by light and its photochemical activity.
• 4.We assessed the compound’s photosensitivity by calculating the radiation frequency and the corresponding absorbed wavelength.
• 5.The Jablonski diagram of propylene glycol was constructed based on computed energy values to illustrate the photophysical transitions.
• 6.The TD-DFT results show that propylene glycol has a relatively large HOMO–LUMO gap (>7 eV), which indicates intrinsic resistance to photoinduced transitions and supports its recognized photostability in pharmaceutical, cosmetic, and industrial applications.

Overall, these findings demonstrate that light exposure induces significant structural and electronic changes in propylene glycol, yet its large HOMO–LUMO gap underlines an inherent photostability that supports its safe and reliable use in pharmaceutical, cosmetic, and industrial formulations. The computational insights presented here should be regarded as complementary to experimental validation, together providing a comprehensive understanding of the photophysical behavior of propylene glycol.

R.K., Prof. Dr. A.K., and O.A. contributed to the conception, data analysis, and writing of the manuscript. R.K. performed the study, analyzed the data, and drafted the manuscript. O.A. carried out additional quantum chemical calculations, revised the manuscript, and assisted in preparing the responses to the reviewers. Prof. Dr. A.K. supervised the work and provided critical revisions. All authors reviewed and approved the final version.

The authors declare no competing financial interest.